Moving foil stripper for a particle accelerator

ABSTRACT

Thin foils for stripping a particle beam are stored on the edge of a disk spinning in the accelerator vacuum. Cutting a foil at one edge releases the foil to project beyond the disk for insertion into the beam at a time determined by controlling the phase of the disk. A wiper removes a spent foil from the disk. The foil release and wiper are operable from a remote location.

United States Patent Gorka, Jr.

MOVING FOIL STRIPPER FOR A PARTICLE ACCELERATOR Inventor: Andrew J. Gorka, Jr., Naperville,

Ill.

Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

Filed: May 30, 1974 Appl. No.1 474,556

US. Cl 328/233, 313/149, 313/359 Int. Cl. H05h 7/00 Field of Search 328/228, 233; 313/359,

References Cited UNlTED STATES PATENTS 12/1960 Stone et al 328/233 X N m a;

[451 Feb. 11, 1975 3,239,707 3/1966 .leppson 313/62 3,353,107 11/1967 Van De Graaff 328/233 FOREIGN PATENTS OR APPLlCATlONS 38-9647 6/1963 Japan 313/149 Primary ExaminerPalmer C. Demeo Attorney, Agent, or Firm.lohn A. Horan; Arthur A. Churm; Donald P. Reynolds [57] ABSTRACT Thin foils for stripping a particle beam are stored on the edge of a disk spinning in the accelerator vacuum. Cutting a foil at one edge releases the foil to project beyond the disk for insertion into the beam at a time determined by controlling the phase of the disk. A wiper removes a spent foil from the disk. The foil release and wiper are operable from a remote location.

9 Claims, 9 Drawing Figures PATENTEUFEBI 11975 @866132 SHEEP-10F 3 20 IO 32 I9 22 PATENTEDFEBI 1 I975 SHEET 2 OF 3 E WHENTED F551 11975 3,866,182

sum 3 or 3 ELECTRICAL SOURCE MOVING FOIL STRIPPER FOR A PARTICLE ACCELERATOR CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to the means of charge exchange known as stripping. It provides improved operation of various kinds of accelerators of charged particles.

Stripping is a well-known technique in the field of charged-particle accelerators. As a part of the process of acceleration, it is often desirable to change the state of charge of the particles in a beam. This is done by causing the particles to collide with other particles. One way of stripping is to introduce molecules of gas into the path of the beam. Another way is to place a thin foil in the path of the beam. The objective is to assure that a high percentage of the number of particles in the beam makes a predetermined number of collisions with stripping molecules, and that a small percentage collides with a different number of stripping molecules. The latter objective requires that, if a foil is used, it be very thin.

Various materials are used to strip. In an example of the instant invention, the stripping atom is carbon which is disposed in the beam path as a very thin foil of an organic polymer. The particular foil used in the practice of the present invention is a thermally stabilized polymer of p-xylene, approximately 3500 A in thickness, formed by vapor deposition. Such foils are usable in thicknesses up to 6000 A. Criteria for foil selection included the following. A material for stripping should comprise atoms of low atomic numbers, preferably at and below the atomic number of carbon. Such a material must withstand the energy of the beam without melting or decomposing. It must have sufficient tensile strength to support itself in pieces that span a cross section larger than the cross section of the beam to be stripped. It must maintain this tensile strength long enough to provide a reasonable period of accelerator operation, of the order of at least an hour, before requiring replacement. It must be self-supporting without a backing and without a framework at all edges. It must withstand the vacuum environment in the interior of a particle accelerator and it must not produce or release particles to comprise a virtual leak.

The stripping foil must be disposed in the beam for stripping at a predetermined time and the means of disposing the foil must permit remote operation. There must be a way to remove and replace foil that has been damaged by radiation, and this way should minimize accelerator downtime. The means for disposing foils must be unaffected by radiation, high vacuum, and the magnetic field of the accelerator.

It is an object of the present invention to provide an improved means of stripping a beam of charged particles.

It is a further object of the present invention to provide a means of disposing a thin film of polyparaxylylene in a beam of charged particles to effect charge exchange.

It is a further object of the present invention to provide a remotely operable means of placing and replacing stripping foils in a particle beam.

It is a further object of the present invention to provide a means of placing stripping foils in a particle beam under conditions of radiation, high vacuum, and high magnetic field.

Other objects will become apparent in the course of a detailed description.

SUMMARY OF THE INVENTION Thin foils of an organic polymer are mounted on the edge of a spinning disk for timed insertion into the beam of a particle accelerator. Collision of the beam with the foil effects stripping. Foils are stored onthe disk and are released at one end one at a time in response to a remote signal. A released foil projects from the disk due to centrifugal force. Spent foils are removed from the edge of the disk, also in response to a remote signal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional side view of an apparatus for the practice of the present invention;

FIG. 2 is a top view of a portion ofa tunnel ofa particle accelerator with the apparatus of the present invention;

FIG. 3 is a partial sectional view along lines 33 of FIG. 1;

FIG. 4 is a sectional view of part of the disk of FIG. 3, taken along section lines 44;

FIG. 5 is a partial schematic end view of thedisk of the present invention taken along section lines 5-5 of FIG. 1;

FIG. 6 is a sectional side view of a portion of the disk of FIG. 5 taken along section lines 66;

FIGS. 7 and 8 are views of a ratchet attached to the disk of FIG. 5; and

FIG. 9 is a view of a portion of the apparatus of FIG. 1 showing the removal of a foil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a sectional side view of an apparatus for the practice of the present invention. In FIG. 1, disk 10 is affixed to shaft 12 which is connected to d-c motor 14 and synchronous motor 16 through rotary coupling 18. Motors 14 and 16 share a common shaft 19. Disk 10 is in the vacuum environment 20 of a particle accelerator, and rotary coupling 18 makes driving contact into vacuum environment 20 from outside it. Do motor 14 is used to start disk 10 rotating with a controlled acceleration over a period of approximately one minute and synchronous motor 16 drives disk 10 at synchronous speed after starting. D-c motor 14 may also vary the load on synchronous motor 16 during synchronous operation. Phasing motor 22 rotates the frame 23 of synchronous motor 16 to adjust its phase and thus the phase of disk 10.

FIG. 1 also shows vacuum port 24 which is used for pumping to maintain vacuum environment 20 within walls 26. Gasketed seal 27 assists in maintaining vacuum environment 20. Heating and indexing transformer 28 has a fixed primary coil 30 and a rotating secondary coil 32 that is attached to disk 10. A wiper 34 is connected to a solenoid 35 which is attached to one wall 26. Frame 36 holds d-c motor 14 and synchronous motor 16 and also provides structural support for wall 26. Frame 36 is mounted on table 38 which is slidably mounted to base 40. I-Iandwheel 42 provides a means to move table 38 with respect to base 40 to position table 38 and hence disk 10.

FIG. 2 is a top view of a portion of a beam tunnel of a particle accelerator with the apparatus of the present invention in position for use. In FIG. 2, beam tunnel 50 is perpendicular to disk so that a foil 48 is inserted into beam tunnel 50, and hence into a beam 51, once per rotation of disk 10. Rotation and control of the timing of foil insertion are controlled by phasing motor 22 of FIG. 1 and synchronous motor 16, as described above.

The disk 10 of FIG. 1 is shown from an end in FIG. 3, which is a sectional view along section line 3-3 of FIG. 1. In FIG. 3, disk 10 is seen along its axis of rotation 44. Several foils 46 are attached at both their ends to disk 10. One foil 48 is attached to disk 10 at one end only, and is maintained in a radially extended position by the centrifugal force associated with the spinning disk 10. The speed and phase of disk 10 are adjusted to place the extended foil 48 in front of beam tunnel 50 for timed insertion into a beam passing through beam tunnel 50. A control signal for maintaining this speed and phase is obtained from reluctance pickup 52, which rotates with disk 10. FIG. 3 also shows wiper 34, which can be moved externally by energizing solenoid 35 of FIG. 1 to graze disk 10 and thus remove a foil 48 for replacement by another foil 46. Thus, one foil 48 is extended at a time for insertion into beam tunnel 50 to strip a beam therein. When it is desired to replace this foil because of radiation damage or any other reason, wiper 34 is moved to the edge of disk 10 to scrape foil 48 free and remove it from disk 10. Another foil 46 is then selected and released at one end to extend beyond the end of disk 10 and thus become a foil 48. The phase of disk 10 is adjusted to place the newly selected foil 48 into the beam at a proper time for stripping.

The means for selecting and releasing a new foil are shown in FIG. 4, which is a sectional view of the disk and wall of the present invention, taken along section lines 4--4 of FIG. 3. In FIG. 4, disk 10 is attached to shaft 12, which is connected to secondary coil 32 of transformer 28. Secondary coil 32 rotates with disk 10, and it is free to move axially on shaft 12. Transformer 28 is completed by primary coil 30, which is attached to wall 26 by supports 54. Wires 56 are connected to feedthrough 58 to make an external connection 60 to electrical source 62, which serves a dual function. When electrical source 62 supplies a d-c voltage through connection 60, feedthrough 58, and wires 56 to primary coil 30, an attractive magnetic force is generated along shaft 12. This force causes motion of secondary coil 32 toward primary coil 30, which steps ratchet mechanism 64 by one step to select a particular foil 46 for release. An a-c voltage of a smaller value, insufficient to operate ratchet mechanism 64, is obtained from electrical source 62 to effect the release of the foil selected by heating it at one end.

Stepping and releasing are better understood by referring to FIG. 5, which is a partial schematic end view of disk 10 taken along section lines 5-5 of FIG. 1. In FIG. 5, secondary coil 32 is shown schematically, together with its terminals 66, which are connected to a pair of wires 68 that lead to a heating element 70. When primary coil 30 of FIG. 4 is excited by the application of an a-c voltage, secondary coil 32 then serves as a secondary source of electrical energy that heats heating element 70. Each foil 46 has previously been secured at an inner end 72 and an outer end 74. Heating element is placed in contact with a foil 46 so that, when heated, it melts through the foil 46. The centrifugal force associated with rotation of disk 10 then flips a foil 46 into the position of foil 48, attached to disk 10 only by the outer end 74. Each foil 46 has its heating element 70. A particular foil 46 is selected for.

release by stepping the terminals 66 of secondary coil 32 to make contact with the pair of wires 68 serving the particular foil 46. In this way the disk 10 serves as a magazine for foils 46.

The means of storing and displaying foils are better seen in FIG. 6, which is a partial sectional view of the disk 10 of FIG. 5, taken along section line 6-6. In FIG. 6, foil 46 is secured to disk 10 by tape 76 at outer end 74 and tape 78 at inner end 72. Wires 68 comprise part of an electrical circuit through heating element 70, which touches foil 46 near inner end 72. When heating element 70 is energized while disk 10 is spinning, foil 46 is melted through and released to extend beyond outer end 74, thus projecting as a foil 48.

The ratchet mechanism 64 is shown in two expanded isometric views in FIGS. 7 and 8. In FIGS. 7 and 8, shaft 80 is connected to and rotates with disk 10 of FIG. 1. Shaft 82 is coaxial with shaft 80 and is connected to and rotates with secondary coil 32 of transformer 28 of FIG. 1. Driving pawl 84 has a driving surface 86 that is offset to form an angle with shafts 80 and 82. Ratcheting pawl 88 has no such offset. Both driving pawl 84 and ratcheting pawl 88 engage cog 90. Driving pawl 84 and ratcheting pawl 88 are connected to shaft 82, and cog 90 is connected to shaft 80.

FIG. 7 shows ratchet mechanism 64 in a relaxed position, evident from the fact that driving surface 86 engages cog 90 near the tip 92 of driving pawl 84. FIG. 8 shows ratchet mechanism 64 in the actuated position, evident from the fact that shaft 80 is closer to shaft 82 than in FIG. 6. Motion of shafts 80 and 82 toward each other causes driving surface 86 to move along cog 90, rotatingcog 90 with respect to driving pawl 84. Ratcheting pawl 88 steps over one tooth of cog 90, and holds cog 90 against turning back, so that driving pawl 84 steps back over a tooth of cog 90 when ratchet mechanism 64 is-returned to the relaxed position by spring 91 of FIG. 4. Thus, by means of this operation of ratchet mechanism 64, shafts 80 and 82 rotate with respect to one another by an angle equal to the angle of one tooth on cog 90.

Ratchet mechanism 64 is actuated by means of electrical source 62 of FIG. 4. When a d-c current is supplied by electrical source 62 to primary coil 30 of transformer 28 of FIG. 4, secondary coil 32 is attracted magnetically toward primary coil 30. This attractive force causes axial motion of secondary coil 32 toward primary coil 30. This, in turn, moves secondary coil 32 with respect to disk 10, producing the ratcheting action described above.

FIG. 9 is a view of a portion of the apparatus of FIG. 1 showing the operation of the wiper 34.1n FIG. 9, disk 10 is rotating, thus extending a foil 48 for stripping. When it is desired to remove foil 48 and replace it with a foil 46 as described, solenoid 35 is energized. This causes wiper 34 to project, striking foil 48 on the way by and tearing foil 48 from disk 10. Foil 48 may be removed because of radiation damage from its use in stripping, or because it did not extend properly, or because it became wrinkled or torn, or for any other reason chosen by an operator.

The present invention has been built and applied to the Booster Accelerator for the Zero Gradient Synchrotron (ZGS) at Argonne National Laboratory. The Booster Accelerator is a converted electron synchrotron which operates at a 30-hertz frequency synchronized to the power lines. The Booster is part of a program to increase the number of particles accelerated per pulse in the 268 by producing improved conditions of injection into the ZGS. It is desirable to inject negative hydrogen ions at energies of the order of 50 MeV into the booster and there to strip them to positive hydrogen ions for acceleration. The injection period is of the order of 200 to 500 msec which necessitates placement of a stripping medium in the beam for that length of time and then its removal. Any particles not stripped will have the wrong sign of electric charge for acceleration and will therefore be lost from the beam. Thus, it is desirable to insure that foil is in the beam during the entire injection period. However, passage through the foil of particles already stripped produces multiple scattering which also results in a loss of particles. These conflicting requirements make it desirable to have a stripping foil in place during and only during injection of negative hydrogen ions at the beginning of the acceleration period.

The stripping medium chosen to meet these requirements is a thermally stabilized polymer of paraxylene formed by vapor depositing this material in a vacuum environment to thicknesses of 4000 Angstroms or less. Such a foil is delicate and is very readily damaged. The spinning disk of the present invention was found satisfactory to store and display such foils. In operation, 56 foils were taped at equally spaced intervals along the edge of a disk having an outer diameter of 36 inches. Each piece of foil was taped at an outer edge of the disk and then the foil was folded back over the tape and over a heating wire for cutoff along a substantially circumferential line spaced radially inward on the disk. The inner end was taped down to maintain the foil in contact with the heating wire. The heating wire was 2'mil Nichrome disposed circumferentially and connected by conductors to the contacts located at the hub of the disk and connectable individually to a heating transformer. Application of a-c power to the transformer heated the Nichrome wire, burning through the foil in a line at the inner end. Rotation of the disk caused the foil to fly out radially and project from the disk. The disk was rotated by a synchronous motor at a speed of 1800 rpm to permit a timed relationship with the 30-Hertz accelerator frequency. The particular beam to be stripped has an elliptical cross-section spanning approximately 2 inches in the horizontal direction and three quarters of an inch in the vertical direction. It was convenient to place the axis of the disk level with the beam and to insert the stripping foil from the top of the beam, thus necessitating a foil length of at least 2 inches. The width of the foil in the direction of motion across the beam is determined by the beam dimension in that direction, the rotational speed of the disk, and the length of time necessary for injection. In this case, a nominal 1 inch in the vertical direction was adequate. However, in order to eliminate the possibility of curling of the foil, it was found desirable to make the foil slightly tapered in shape with the wider dimension at the disk. For this reason, the foils used were 1 inch wide at the edge away from the disk and tapered symmetrically to 1 /2 inches in width at the edge of the disk.

Experiments using comparable foils in stripping a proton beam indicated that approximately 5 X 10 particles passed through the foil before destruction occurred. The pulses expected in the present application will have a typical number of the order of IO particles per pulse. When used to supply pulses for acceleration in the Z68, a foil should therefore be expected to last of the order of 2 hours before requiring replacement. When this is necessary, the solenoid is engaged to wipe the spent foil from the edge of the disk, a d-c signal is applied to cause the ratcheting mechanism to advance to the next foil and the next foil is released by heating the Nichrome heater wire.

The disk was made for rapid removal from the shaft so that a standby disk can be loaded with foils for relatively rapid replacement when all 56 foils are spent. This is a shutdown job, since it involves losing the vacuum environment in which the disk is rotated. The time to change disks is of the order of an hour.

The phase of the synchronous motor driving the disk is critical to place the foil in the proper place at the proper time. This placement is accomplished by rotating the frame of the synchronous motor by a stepping motor driving a worm gear attached to the motor frame. Two means of checking the foil placement were used. One is a television monitor viewing an image of the foil as the foil is illuminated by a stroboscopic light. The other is a reluctance pickup placed on the disk and thus timed to a particular location on the disk. Phase information is readily obtained from this signal by use of a scaler to determine the time delay between the fixed position and each individual foil position. The stepping motor was used because it was believed desirable to maintain an accuracy of the order of 10 microseconds in placing the foil. This corresponds to a placement accuracy of O.l or 1 part in 3600. This accuracy is exceeded by a stepping motor making 200 discrete steps per revolution and driving a worm gear having a reduction ratio of to 1. An additional control of the accuracy of foil placement is achieved by using the d-c starting motor as a variable loading element after the synchronous motor has taken over the driving function. The d-c motor can be driven to reduce the load on the synchronous motor and facilitate maintaining synchronism. Alternatively, the d-c motor can be used as a generator and thus a load on the synchronous motor if necessary to damp out hunting oscillations.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for timed insertion of thin stripping foils into the beam of a particle accelerator comprising:

a circular disk disposed in a vacuum environment in a plane perpendicular to the beam and substantially tangent to the beam;

a rotating drive connected to the circular disk to rotate said disk, said rotating drive including means for adjusting the phase of rotation of said circular disk relative to the beam;

a stripping foil connected to the circular disk at a first edge of said foil near the circumference of the disk and at a second edge of said foil radially inward from said first edge; and

means for releasing said foil near said second edge while rotating said disk, whereby release of said foil and adjustment of said phase of rotation effects timed insertion of said foil into said beam.

2. The apparatus of claim 1 wherein the rotating drive comprises:

a disk shaft connected to the disk on the axis of the disk;

a motor shaft coaxial with the disk shaft and disposed outside the vacuum environment;

means connected to the disk shaft and the motor shaft to couple said shafts in a rotating linkage;

a synchronous a-c motor coupled to said motor shaft;

means connected to said synchronous a-c motor to vary the phase of rotation thereof.

3. The apparatus of claim 1 wherein said means for releasing said foil comprise:

an electrical heater wire connected to said disk and to said stripping foil at said second edge;

a source of a-c electrical energy;

an electrical transformer having a primary winding connected electrically to said source and a secondary winding connected electrically to said electrical heater wire to heat said heater wire and thereby release said foil,

4. The apparatus of claim 1 wherein said disk includes a plurality of stripping foils, each of said foils having a first edge connected to said disk near the circumference thereof and a second edge connected to said disk at a point radially inward from said first edge, further wherein said apparatus includes means for releasing each of said plurality of stripping foils near said second edges while rotating said disk.

5. The apparatus of claim 4 wherein said means for releasing said foil sections one at a time comprise:

a plurality of electrical heater wires each connected to said disk and each connected to one of said foil sections at a second edge thereof;

a source of a-c electrical energy;

a source of d-c electrical energy;

an electrical transformer having a secondary winding that is rotatable about an axis while maintaining transformer coupling with a primary winding, said primary winding secured in a stationary position, said secondary winding connected by ratcheting means to said rotating disk to rotate with said disk and to move axially with respect to said primary winding in response to a dc current from said source of d-c electrical energy to operate said ratcheting means and index said secondary winding with respect to said disk to connect to a particular one of said plurality of heater wires to establish a path therethrough for a-c current from said source of a-c electrical energy to heat and melt through said one of said foil sections and thereby release said foil.

6. The apparatus of claim -1 comprising in addition means for removing said released foil from said disk while rotating said disk.

7. The apparatus of claim 6 wherein said means for removing said released foil comprise:

a stationary solenoid disposed at the periphery of said disk; and

a plunger connected to the solenoid and extendable by actuating the solenoid to project into the path of said rotating released foil and thereby catch said foil and tear said foil from said rotating disk.

8. The apparatus of claim 1 wherein the stripping foil is a vapor-deposited polymer of p-xylene (polyparaxylylene) having a nominal thickness less than 4000 Aug stroms and having a surface cross-section larger than the cross-section of the beam.

9. An apparatus for making timed insertion of stripping foils into the beam of a particle accelerator comprising:

a disk disposed in the vacuum environment of the particle accelerator in a plane perpendicular to the beam and tangent to the beam;

a rotary coupling connected to said disk to couple into the vacuum environment;

a shaft connected to said rotary coupling;

a d-c motor located outside the vacuum environment and connected to said shaft to effect thereon rotation of said disk;

a synchronous motor connected to said shaft to effect synchronous rotation of said shaft and hence of said disk;

a phasing motor connected to said synchronous motor to vary the phase thereof;

a plurality of foils of poly-paraxylylene, each attached to the disk at an outer edge near the circumference of the disk and at an inner edge located radially inward on the disk from the outer edge;

a plurality of heating wires connected to the disk, each heating wire touching one of said plurality of foils near the inner edge of the foil;

21 source of electrical energy;

remotely controllable indexing means for establishing an electrical connection to a selected one of said plurality of heater wires and to said source of electrical energy and releasing one of said plurality of heater wires; and

remotely controllable wiping means disposed in the vacuum environment to wipe the circumference of the disk and thereby remove a foil from the disk. 

1. An apparatus for timed insertion of thin stripping foils into the beam of a particle accelerator comprising: a circular disk disposed in a vacuum environment in a plane perpendicular to the beam and substantially tangent to the beam; a rotating drive connected to the circular disk to rotate said disk, said rotating drive including means for adjusting the phase of rotation of said circular disk relative to the beam; a stripping foil connected to the circular disk at a first edge of said foil near the circumference of the disk and at a second edge of said foil radially inward from said first edge; and means for releasing said foil near said second edge while rotating said disk, whereby release of said foil and adjustment of said phase of rotation effects timed insertion of said foil into said beam.
 2. The apparatus of claim 1 wherein the rotating drive comprises: a disk shaft connected to the disk on the axis of the disk; a motor shaft coaxial with the disk shaft and disposed outside the vacuum environment; means connected to the disk shaft and the motor shaft to couple said shafts in a rotating linkage; a synchronous a-c motor coupled to said motor shaft; means connected to said synchronous a-c motor to vary the phase of rotation thereof.
 3. The apparatus of claim 1 wherein said means for releasing said foil comprise: an electrical heater wire connected to said disk and to said stripping foil at said second edge; a source of a-c electrical energy; an electrical transformer having a primary winding connected electrically to said source and a secondary winding connected electrically to said electrical heater wire to heat said heater wire and thereby release said foil.
 4. The apparatus of claim 1 wherein said disk includes a plurality of stripping foils, each of said foils having a first edge connected to said disk near the circumference thereof and a second edge connected to said disk at a point radially inward from said first edge, further wherein said apparatus includes means for releasing each of said plurality of stripping foils near said second edges while rotating said disk.
 5. The apparatus of claim 4 wherein said means for releasing said foil sections one at a time comprise: a plurality of electrical heater wires each connected to said disk and each connected to one of said foil sections at a second edge thereof; a source of a-c electrical energy; a source of d-c electrical energy; an electrical transformer having a secondary winding that is rotatable about an axis while maintaining transformer coupling with a primary winding, said primary winding secured in a stationary position, said secondary winding connected by ratcheting means to said rotating disk to rotate with said disk and to move axially with respect to said primary winding in response to a d-c current from said source of d-c electrical energy to operate said ratcheting means and index said secondary winding with respect to said disk to connect to a particular one of said plurality of heater wires to establish a path therethrough for a-c current from said source of a-c electrical energy to heat and melt through said one of said foil sections and thereby release said foil.
 6. The apparatus of claim 1 comprising in addition means for removing said released foil from said disk while rotating said disk.
 7. The apparatus of claim 6 wherein said means for removing said released foil comprise: a stationary solenoid disposed at the periphery of said disk; and a plunger connected to the solenoid and extendable by actuating the solenoid to project into the path of said rotating released foil and thereby catch said foil and tear said foil from said rotating disk.
 8. The apparatus of claim 1 wherein the stripping foil is a vapor-deposited polymer of p-xylene (polyparaxylylene) having a nominal thickness less than 4000 Angstroms and having a surface cross-section larger than the cross-section of the beam.
 9. An apparatus for making timed insertion of stripping foils into the beam of a particle accelerator comprising: a disk disposed in the vacuum environment of the particle accelerator in a plane perpendicular to the beam and tangent to the beam; a rotary coupling connected to said disk to couple into the vacuum environment; a shaft connected to said rotary coupling; a d-c motor located outside the vacuum environment and connected to said shaft to effect thereon rotation of said disk; a synchronous motor connected to said shaft to effect synchronous rotation of said shaft and hence of said disk; a phasing motor connected to said synchronous motor to vary the phase thereof; a plurality of foils of poly-paraxylylene, each attached to the disk at an outer edge near the circumference of the disk and at an inner edge located radially inward on the disk from the outer edge; a plurality of heating wires connected to the disk, each heating wire touching one of said plurality of foils near the inner edge of the foil; a source of electrical energy; remotely controllable indexing means for establishing an electrical connection to a selected one of said plurality of heater wires and to said source of electrical energy and releasing one of said plurality of heater wires; and remotely controllable wiping means disposed in the vacuum environment to wipe the circumference of the disk and thereby remove a foil from the disk. 